In order to treat cancer with radiation, it is highly desirable to maximize the dose delivered to the target volume containing the tumor which is visible with various imaging modalities and some predetermined margin while sparing normal tissue.
Many mechanical configurations of radiation therapy machines and the associated radiation sources have been developed since Roentgen discovered X-Rays. Modern radiation therapy systems use relatively high energy beams of radiation from radioactive isotopes, particle beam accelerators, or electron beam X-Ray generators. The X-Ray generators can employ either high voltage direct current or RF driven linear accelerators (LINACs). The conventional radiation therapy system uses a LINAC to generate an electron beam with between 4 and 22 MeV of energy at low current. The electron beam strikes a high-Z target, typically tungsten, and generates penetrating x-rays. The beam is shaped and delivered to the target volume from one or more directions. The overlapping dose at the target volume is usually higher than the dose at the surface from any one delivery angle. The skin is sensitive to radiation, so it is desirable to limit the skin dose to minimize complications. If more delivery angles are used, the surface dose can be spread out and minimized with respect to the dose delivered to the target volume. A significant fraction of all radiation therapy treatments are employed to treat breast cancer with very good success. A typical general purpose radiation therapy system is designed to treat virtually all anatomical sites with some trade-offs being made in the design in order to make a universally applicable machine. A linear accelerator rotates about a horizontal axis, around a virtual point called the isocenter that intersects with the beam axis. A typical source to axis distance, or source to isocenter distance, SAD is 100 centimeters. The treatment couch rotates about a vertical axis intersecting with the same isocenter and including three additional Cartesian motions for patient alignment. The external dimensions and geometry of different linear accelerators vary, resulting in different available treatment angles, which can be limiting to couch position and gantry rotation.
Currently most of the breast cancer patient population is treated in the supine (lying on the back) position, which does not allow access from more than a few angles. The supine position is also inferior due to gravitational forces compressing the breast against the chest. In addition, breast motion resulting from breathing creates inaccuracies in locating the beam with respect to the target volume.
Alternatively, a prone position radiation therapy is used for access to the breast. This method is implemented with a table top attachment that works with a standard linear accelerator. This embodiment helps to reduce target motion associated with breathing and create a better separation of the target tissue with respect to the chest wall and other critical structure. However, this embodiment still only allows access to a few angles, typically two.